Actions: Add flake update action

.forgejo/workflows/update.yaml

@@ -0,0 +1,19 @@
+name: scheduled flake.lock update
+on:
+  schedule:
+    - cron: "0 0 * * 0,2,4"
+  workflow_dispatch: {}
+
+jobs:
+  flake-lock-update:
+    runs-on: nixos-latest
+    steps:
+      - run: nix-env -iA nixpkgs.nodejs_20 nixpkgs.gnused nixpkgs.coreutils
+      - run: mkdir -p ~/.config/nix && echo "experimental-features = nix-command flakes" > ~/.config/nix/nix.conf
+      - uses: actions/checkout@v4
+        with:
+          ref: 'main'
+      - uses: actions/nix-flake-update-pr.git@main
+        with:
+          token: ${{ secrets.FORGEJO_PR_TOKEN }}
+          author-email: flake@flyinggecko.org